Method and apparatus for steam dealkylation in a plant for the catalytic reforming of hydrocarbons

ABSTRACT

A method and apparatus for treating a fraction consisting predominantly of hydrocarbons having at least six carbon atoms (C 6+  fraction) as produced in a plant for the catalytic reforming of hydrocarbon-containing feedstock, is disclosed. The C 6+  fraction is taken for steam dealkylation where the useable products benzene and hydrogen are produced.

This application claims the priority of German Patent Documents No. 10 2006 038 892.5, filed Aug. 18, 2006, and No. 10 2006 058 534.8, filed Dec. 12, 2006, the disclosures of which are expressly incorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method for treating a fraction consisting predominantly of hydrocarbons having at least six carbon atoms (C₆₊ fraction) as produced in a plant for the catalytic reforming of hydrocarbon-containing starting material and an apparatus to carry out the method.

Heavy naphtha is produced primarily in a plant for the catalytic reforming of hydrocarbon-containing feedstock, such as is produced, for example, in crude oil distillation.

The heavier naphtha such as is produced in crude oil distillation, contains principally iso- and n-paraffins, napthenes and aromatics having predominantly six to twelve carbon atoms, where the percentage of aromatics may also be very low and depends on the starting material. In the prior art, the heavier naphtha first undergoes desulfurization in which hydrogen is consumed and hydrogen sulfide is created and is then taken as starting material to catalytic reforming. In the catalytic reforming, principally the existing paraffins and napthenes are converted into aromatics in the presence of a catalyst, where hydrogen and light hydrocarbons are created/formed as by-products. These by-products are separated from the reaction products of the catalytic reforming so that a fraction consisting predominantly of hydrogen and hydrocarbons having up to five carbon atoms and a fraction consisting predominantly of hydrocarbons having at least six carbon atoms (C₆₊ fraction) is produced. This C₆₊ fraction contains aromatics as an economically usable product, primarily benzene, which are used as starting material for the synthesis of numerous plastics and to increase the knock resistance of gasoline.

In order to obtain the economically usable products from the C₆₊ fraction, principally benzene, and to make the yield as large as possible, the following method is used in accordance with the prior art. By means of fluid-fluid extraction, the linear hydrocarbons are separated and processed further as raffinate, for example the raffinate can be returned to the starting material for catalytic reforming. The C₆₊ fraction freed from the linear hydrocarbons now contains primarily aromatics having six to eight carbon atoms and is separated into a fraction consisting predominantly of hydrocarbons having six or seven carbon atoms (principally benzene and toluene) and into a fraction consisting predominantly of hydrocarbons having eight carbon atoms (primarily xylene). The fraction consisting predominantly of hydrocarbons having at least eight carbon atoms is taken as starting material to a process for extracting para-xylene. Benzene is extracted from the fraction consisting predominantly of hydrocarbons having six or seven carbon atoms before this fraction is taken as starting material to a process for hydro-dealkylation.

A method of this kind for hydro-dealkylation is described, for example, in WO2005071045. The hydrocarbons are contacted with hydrogen in the presence of a catalyst at a temperature of 400° C. to 600° C. and a pressure between 20 bar and 40 bar, where the hydrogen is present in a molar excess of three to six times the hydrocarbons. Under these conditions the alkyl groups are split off from the specific alkylated aromatics (for example, toluene or xylene) so that benzene and the specific alkanes (for example, methane and ethane) form.

The consumption of hydrogen in the hydro-dealkylation of the hydrocarbons has a negative effect on the economics of this method from the prior art for extracting benzene.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates an embodiment of an apparatus in accordance with the principles of the present invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

In accordance with the invention, with respect to the method, the C₆₊ fraction is subjected to steam dealkyation where mainly the two utilizable products benzene and hydrogen are produced along with reaction products such as carbon monoxide and carbon dioxide.

The basic idea of the invention is to carry out the dealkylation of the alkylated aromatics while generating benzene with the aid of steam dealkylation. Steam dealkylation requires only low-cost steam as the starting material and produces the valuable by-product hydrogen in addition to the desired quality product benzene.

The C₆₊ fraction used in the steam dealkylation contains primarily:

-   -   a) aromatic hydrocarbons having six to ten carbon atoms,     -   b) cyclic paraffins (cycloalkenes) having five to ten carbon         atoms,     -   c) iso- and n-paraffins having five to ten carbon atoms,     -   d) alkenes having six to ten carbon atoms, or         any mixture of the preceding, in which the exact composition of         the mixture depends on the composition of the specific heavier         naphtha which is taken as starting material for catalytic         reforming. The method in accordance with the invention is         suitable for each of the compounds of the C₆₊ fractions         described.

The hydrocarbons from the C₆₊ fraction advantageously react with steam in the gas phase with the introduction of heat on a solid catalyst. The gaseous C₆₊ fraction is dealkylated by the presence of gaseous water (steam) on a catalyst with the constant introduction of heat, whereby the desired products benzene and hydrogen are produced in addition to carbon monoxide, carbon dioxide and additional by-products.

Preferably the heat required for the dealkylation reaction is generated from combustion of a starting material with air. It proves to be particularly advantageous to use gaseous reaction by-products from the steam dealkylation, specifically carbon monoxide and methane as the starting material for combustion with air. A part of the gaseous reaction by-products from the steam dealkylation, in particular carbon monoxide and methane, is combustible and can thus serve as starting material for combustion to generate the required reaction heat. This saves heating gas and this otherwise unused part of the reaction products is employed usefully.

The gaseous reaction products, following compression, are expediently separated by way of pressure swing adsorption into gaseous hydrogen and gaseous reaction by-products, specifically carbon monoxide, carbon dioxide and methane. The valuable by-product hydrogen is also present in gaseous form and can be employed much more usefully than in combustion. By means of pressure swing adsorption with prior compression, the hydrogen can easily be separated from the combustible gaseous reaction by-products which can serve as starting material in the combustion.

The flue gases generated during combustion are advantageously cooled by means of a heat exchanger while heating the starting materials for steam dealkylation. By using the heat of the flue gases to pre-heat the starting materials (C₆₊ fraction and steam) for steam dealkylation, the heat that has to be brought in to maintain the required temperatures for the steam dealkylation is reduced. This achieves an economical use of energy resources.

The C₆₊ fraction and the steam are advantageously taken past the solid catalyst in pipes, preferably from top to bottom, with the catalyst being located inside the pipes. Heat is expediently brought to the pipes from the outside. The heat required for the dealkylation reaction is advantageously transferred to the pipe by electromagnetic radiation, thermal radiation and/or convection. The actual dealkylation reaction takes place inside the pipes where the catalyst is located. The two components in the reaction (C₆₊ fraction and steam) are taken from top to bottom through the pipes filled with the catalyst. The heat required for the dealkylation reaction is generated outside the pipes and transferred to the pipe by the mechanisms named from which the heat is transferred by means of conduction and convection into the interior of the pipes where the reaction is taking place.

Preferably a solid catalyst of a porous carrier material is used, in particular γ-Al₂O₃, MgAl spinel and/or Cr₂O₃, and an active component on the surface of the carrier material, in particular Rh with 0.1-1.0% loading by weight and/or Pd with 0.2-2.0% loading by weight.

The steam dealkylation is advantageously performed at a temperature of 400° C. to 600° C., preferably 450° C. to 550° C., particularly preferably 480° C. to 520° C. and at a pressure of 1 to 15 bar, preferably 1.2 to 10 bar, particularly preferably 1.5 to 8 bar.

The steam dealkylation is expediently performed at a molar quotient of steam to hydrocarbons which lies in the range from 1 to 20, preferably from 2 to 15, when it enters the reactor. In another embodiment of the invention, the steam dealkylation is performed at a molar quotient of steam to hydrocarbons which lies in the range from 3 to 12, preferably from 5 to 10 when it enters the reactor. Generally the steam dealkylation is performed with a molar excess of water, where the exact ratio in the different embodiments of the inventions depends on the precise composition of the C₆₊ fraction.

It proves advantageous to subject the C₆₊ fraction before steam dealkylation to a process to convert dienes and styrenes, where specifically hydrating methods consuming hydrogen are employed. In another embodiment of the invention, the C₆₊ fraction is separated before steam dealkylation from a fraction of hydrocarbons having at least six carbon atoms where the fraction of hydrocarbons having at least six carbon atoms is subjected to a process to convert dienes and styrenes, specifically a hydrating process which consumes hydrogen. By employing the hydrating methods, any diolefins present in the C₆₊ fraction are converted into their corresponding olefins, just as components containing sulfur, nitrogen and oxygen can be converted and removed. Deactivation of the catalyst is reduced and the life of the catalyst is clearly increased.

The reaction products from the steam dealkylation are preferably cooled and separated in a 3-phase separation into gaseous reaction products, hydrocarbons and water. The reaction products coming from the steam dealkylation contain not only the desired quality products benzene and hydrogen but also reaction products such as carbon monoxide and carbon dioxide and reaction by-products. To obtain the desired quality products, the reaction products must be separated. This is done by way of a 3-phase separation of the cooled reaction products into gaseous reaction products, in particular hydrogen, carbon monoxide, carbon dioxide and methane, into hydrocarbons, in particular benzene, and into water.

The hydrogen generated in the steam dealkylation of the C₆₊ fraction is expediently fed completely or partially into the starting material for the hydrogen-consuming processes. The hydrogen generated in the steam dealkylation can be used entirely or partially for the hydrogen-consuming processes described in the previous section so that the need for hydrogen to be supplied externally is minimized.

In a further embodiment of the invention the hydrogen produced in the steam dealkylation of the C₆₊ fraction is taken as starting material to any process consuming hydrogen in the oil refinery, preferably to a process for converting and removing sulfur-containing components or to a process for reforming a hydrocarbon-containing starting material by means of hydrogen.

Reduction of the sulfur content in the C₆₊ fraction to below 10 ppm, preferably to below 3 ppm, particularly preferably to below 1 ppm, before steam dealkylation proves advantageous for a good yield of the desired reaction product benzene.

Preferably the benzene is separated from the hydrocarbons of the reaction products through rectification. Following rectification, the benzene advantageously undergoes adsorptive fine cleaning to dry and remove the trace components, where the benzene is directed across an adsorbent on which the trace components, as opposed to benzene, are adsorbed. By applying the inventive process, the benzene can be extracted from the reaction products by simple rectification and processed further or marketed. Expensive rectification or extra rectification as when applying a process in accordance with the prior art is not necessary, thus reducing investment and process costs.

Advantageously components boiling close to benzene or components forming azeotropes in the C₆₊ fraction are converted by the steam dealkylation. All reaction products boiling heavier than benzene from rectification, consisting predominantly of non-converted starting materials from the steam deakylation are expediently returned to steam dealkylation through optional hydration as starting material. In another embodiment of the invention, all reaction products boiling heavier than benzene from rectification, consisting predominantly of non-converted starting materials from steam dealkylation are returned for hydration of the C₆₊ fraction or hydration of a fraction consisting predominantly of hydrocarbons having at least five carbon atoms prior to steam dealkylation. By returning the non-converted starting materials for hydration or for steam dealkylation, circulation is achieved without losing valuable starting materials.

In a further embodiment of the invention, the linear hydrocarbons are separated from the C₆₊ fraction prior to steam dealkylation by means of fluid-fluid extraction, whereby the linear hydrocarbons are returned to the starting material for catalytic reforming.

In another embodiment of the invention, prior to steam dealkylation a fraction consisting predominantly of hydrocarbons having at least eight carbon atoms (C₈₊ fraction) is separated by distillation from the C₆₊ fraction, where the separated C₈₊ fraction is taken to a process for extracting para-xylene or gasoline. Following separation of the C₈₊ fraction, benzene is advantageously separated from the C₆₊ fraction prior to the steam dealkylation. Through the separation of the C₈₊ fraction and the removal of benzene, the C₆₊ fraction now contains predominantly toluene which is effectively converted into benzene by the application of the method in accordance with the invention.

Concerning the apparatus, the object of the invention is achieved by the apparatus comprising an oven 100 with a furnace 110 and pipes 120 located in the furnace. The actual steam dealkylation takes place in the pipes which in turn are located in the furnace of the oven where the heat required for steam dealkylation can be generated.

The pipes are advantageously installed vertically in the furnace and have heat expansion compensating elements 130 at the lower and/or upper end. The heat expansion compensating elements at the lower and/or upper end of the vertical pipes prevent mechanical stress from temperature differences which can lead to increased wear of the pipes.

Each pipe expediently has a supply for the C₆₊ fraction and the steam, 122, 124, respectively, and an outlet 126 for the reaction products.

It similarly proves advantageous that each pipe is filled on the inside with a catalyst, where the catalyst consists of a porous carrier material, in particular γ-Al₂O₃, MgAl spinel and/or Cr₂O₃ and an active component on the surface of the carrier material, in particular Rh with 0.1-1.0% loading by weight and/or Pd with 0.2.-2.0% loading by weight.

Preferably the oven has at least one burner 102 on the wall, the ceiling and/or the floor. The pipes are expediently suitable for an internal pressure of 1 to 15 bar, preferably 1.2 to 10 bar, particularly preferably 1.5 to 8 bar, and for use in an oven with flame temperatures of up to 1400° C.

The present invention is successful specifically in creating an economical alternative to the prior art for treating a C₆₊ fraction. Through the application of the inventive method and the inventive apparatus, the valuable by-product hydrogen is generated in addition to the usable product benzene.

The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof. 

1. A method for treating a fraction consisting predominantly of hydrocarbons having at least six carbon atoms (C₆₊ fraction) as produced in a plant for catalytic reforming of hydrocarbon-containing feedstock, wherein the C₆₊ fraction undergoes steam dealkylation, where two usable product materials benzene and hydrogen are produced in addition to reaction products such as carbon monoxide and carbon dioxide.
 2. The method according to claim 1, wherein the C₆₊ fraction contains: a) aromatic hydrocarbons having six to ten carbon atoms; b) cyclic paraffins (cycloalkenes) having five to ten carbon atoms; c) iso- and n-paraffins having five to ten carbon atoms; d) alkenes having six to ten carbon atoms; or any mixture of the aforementioned.
 3. The method according to claim 1, wherein the hydrocarbons from the C₆₊ fraction react with water in a gas phase with addition of heat to a solid catalyst.
 4. The method according to claim 1, wherein heat required for the dealkylation reaction is generated by combustion of a starting material with air.
 5. The method according to claim 1, wherein gaseous reaction products from the steam dealkylation are separated following compression by way of pressure swing adsorption into gaseous hydrogen and gaseous reaction by-products, specifically carbon monoxide, carbon dioxide and methane.
 6. The method according to claim 5, wherein the gaseous reaction by-products from the steam dealkylation, specifically carbon monoxide and methane, are used as starting material for the combustion with air.
 7. The method according to claim 1, wherein flue gases generated during combustion are cooled by a heat exchanger while heating starting materials for the steam dealkylation.
 8. The method according to claim 1, wherein the C₆₊ fraction and the steam are conducted in pipes, from top to bottom, past a solid catalyst, where the catalyst is on an inside of the pipes.
 9. The method according to claim 8, wherein heat is brought to the pipes from outside.
 10. The method according to claim 9, wherein the heat required for the dealkylation reaction is transferred to the pipes by electromagnetic radiation, thermal radiation and/or convection.
 11. The method according to claim 1, wherein a solid catalyst of a porous carrier material is used, specifically γ-Al₂O₃, MgAl spinel and/or Cr₂O₃ and an active component on a surface of the carrier material, in particular Rh with 0.1-1.0% loading by weight and/or Pd with 0.2.-2.0% loading by weight.
 12. The method according to claim 1, wherein the steam dealkylation is performed at a temperature of 400° C. to 600° C., preferably 450° C. to 550° C., particularly preferably 480° C. to 520° C.
 13. The method according to claim 1, wherein the steam dealkylation is performed at a pressure from 1 to 15 bar, preferably 1.2 to 10 bar, particularly preferably 1.5 to 8 bar.
 14. The method according to claim 1, wherein the steam dealkylation is performed at a molar quotient of steam to hydrocarbons in a range from 1 to 20, preferably from 2 to 15, when it enters a reactor.
 15. The method according to claim 1, wherein the steam dealkylation is performed at a molar quotient of steam to hydrocarbons which is in a range from 3 to 12, preferably from 5 to 10, when it enters a reactor.
 16. The method according to claim 1, wherein the C₆₊ fraction undergoes a process prior to the steam dealkylation to convert dienes and styrenes where in particular hydrating methods are employed involving consumption of hydrogen.
 17. The method according to claim 1, wherein the C₆₊ fraction undergoes a process prior to the steam dealkylation to convert and to remove components containing sulfur, nitrogen and/or oxygen, in which specifically hydrating processes involving consumption of hydrogen are employed.
 18. The method according to claim 1, wherein the reaction products from the steam dealkylation are cooled and separated into gaseous reaction products, hydrocarbons and water in a 3-phase separation.
 19. The method according to claim 16, wherein the hydrogen produced in the steam dealkylation of the C₆₊ fraction is fed completely or partially into a starting material for the processes involving the consumption of hydrogen.
 20. The method according to claim 17, wherein the hydrogen produced in the steam dealkylation of the C₆₊ fraction is fed completely or partially into a starting material for the processes involving the consumption of hydrogen.
 21. The method according to claim 1, wherein the hydrogen produced in the steam dealkylation of the C₆₊ fraction is fed as starting material to a process consuming hydrogen in an oil refinery, preferably into a process to convert and remove components containing sulfur or a process to split hydrocarbon-containing starting material via hydrogen.
 22. The method according to claim 1, wherein a sulfur content in the C₆₊ fraction is reduced to below 10 ppm, preferably below 3 ppm, particularly preferably below 1 ppm prior to the steam dealkylation.
 23. The method according to claim 1, wherein the benzene is separated from the hydrocarbons by way of rectification of the reaction products.
 24. The method according to claim 23, wherein the benzene undergoes adsorptive fine cleaning following rectification to dry and remove trace components, where the benzene is passed across an adsorbent on which the trace components are adsorbed.
 25. The method according to claim 1, wherein components boiling close to benzene or forming azeotropes in the C₆₊ fraction are converted by steam dealkylation.
 26. The method according to claim 23, wherein all heavier boiling reaction products than benzene from rectification, consisting predominantly of non-converted feedstocks from the steam dealkylation are returned to the steam dealkylation as feedstock via optional hydration.
 27. The method according to claim 23, wherein all heavier boiling reaction products than benzene from rectification consisting predominantly of non-converted feedstocks from the steam dealkylation are returned prior to steam dealkylation for hydration of the C₆₊ fraction or for hydration of a fraction consisting predominantly of hydrocarbons having at least five carbon atoms.
 28. The method according to claim 1, wherein linear hydrocarbons are separated from the C₆₊ fraction prior to steam dealkylation by means of liquid-liquid extraction.
 29. The method according to claim 1, wherein a fraction consisting predominantly of hydrocarbons having at least eight carbon atoms (C₈₊ fraction) is separated by distillation from the C₆₊ fraction prior to steam dealkylation where the C₈₊ fraction is taken as feedstock to a process to extract paraxylene.
 30. The method according to claim 29, wherein following separation of the C₈₊ fraction, benzene is separated from the C₆₊ fraction prior to the steam dealkylation.
 31. An apparatus for treating a fraction consisting predominantly of hydrocarbons having at least six carbon atoms (C₆₊ fraction) as produced in a plant for catalytic reforming of hydrocarbon-containing feedstock, wherein the apparatus includes an oven with a furnace and pipes located in the furnace.
 32. The apparatus according to claim 31, wherein the pipes are mounted vertically in the furnace and have heat expansion compensating elements at a lower and/or an upper end.
 33. The apparatus according to claim 31, wherein each pipe has a supply for the C₆₊ fraction and the steam and an outlet for the reaction products.
 34. The apparatus according to claim 31, wherein each pipe is filled on an inside with a catalyst, where the catalyst consists of a porous carrier material, specifically γ-Al₂O₃, MgAl spinel and/or Cr₂O and an active component on a surface of the carrier material, in particular Rh with 0.1-1.0% loading by weight and/or Pd with 0.2.-2.0% loading by weight.
 35. The apparatus according to claim 31, wherein the oven has at least one burner on a wall, a ceiling and/or a floor.
 36. The apparatus according to claim 31, wherein the pipes are suitable for an internal pressure of 1 to 15 bar, preferably 1.2 to 10 bar, particularly preferably 1.5 to 8 bar, and for use in an oven with flame temperatures of up to 1400° C.
 37. A method of extracting benzene from a hydrocarbon having at least six carbon atoms, comprising the steps of: producing the hydrocarbon having at least six carbon atoms in a plant for catalytic reforming of hydrocarbon-containing feedstock; subjecting the hydrocarbon having at least six carbon atoms to steam dealkylation; and producing benzene from the steam dealkylation.
 38. The method according to claim 37, further comprising the step of producing hydrogen from the steam dealkylation. 